Monitoring Changes in the Overburden – Some Observations from Using Various 2D and 3D Seismic Time Lapse Data Sets

Abstract

In 1989 a major underground blow out developed when well 2/4-14 was drilled into a target reservoir at 4.5 km depth in the North Sea. 2D seismic data was acquired both prior to and during the underground blow out. In addition to these data, we use two 3D seismic data sets, one acquired 2 years after the blow out, in 1991 and another acquired in 2005. By comparing various data, we can analyse both short term and especially long term movements of hydrocarbon flows in the overburden. Since there is significant amount of shallow gas in the area, it is a challenge to discriminate between in-situ shallow gas and gas originating from the underground flow. We find that the combination of slightly dipping shallow glacial sand layers and ice berg plough marks serve as a trap for shallow gas. This can be mapped from the 3D seismic data, as well as flow of gas due to the underground blow out.